With rising world-wide energy demands and soaring prices of fossil fuels, interest in renewable energy sources has increased. Among these, solar photovoltaic (PV) energy has seen a rapid growth in the last few years.
FIG. 1 shows a schematic drawing of a conventional photovoltaic (PV) system 10. The system 100 includes a solar array 1, also commonly referred to as a “PV array.” The PV array 1 typically includes multiple PV modules electrically connected together in series. Each PV module in turn typically includes multiple PV cells (also electrically connected together in series). The DC output voltage 2 of the solar array 1 is controlled by a maximum power point tracking (MPPT) apparatus 3 to obtain optimum power extraction from the solar array 1. The maximum power point (MPP) of the array 1 is the operating point of output current and voltage at which the array produces the highest amount of power, and this point changes with temperature and irradiation; accordingly, the MPPT apparatus 3 dynamically adjusts the operating point of the solar array 1 to track these changes. The DC output voltage 4 of the MPPT typically is then fed to an inverter 5, which provides an AC voltage to the power grid 6. In some conventional implementations, the function of the MPPT apparatus 3 is integrated into the inverter 5. However, in other implementations an MPPT apparatus 3 may be implemented separately from the inverter 5.
FIG. 2 shows an exemplary conventional PV module 31, which may include from thirty-six to seventy-two series-connected PV cells 32. The right side of FIG. 2 shows the respective circuit symbols for a PV module 31 and a PV cell 32. Because the cells 32 are all connected in series, the module output current is limited by the weakest cells. The output current of each cell 32 varies strongly with irradiation; FIGS. 3a and 3b respectively illustrate that the output power and current of a PC cell both are significantly higher when a PV cell receives full sunlight (e.g., 1 kW/m2) than when it receives 25% of full sunlight. The current also changes with manufacturing lot (sometimes also within a lot), temperature and age. The resulting problem, called cell-current mismatch, is a common phenomenon which reduces power yield in PV modules.
The most severe effects of cell-current mismatch often are seen when PV modules experience different irradiation levels (typically due to partial shading). The shaded cells are reverse biased by the other series-connected cells, and can be driven into reverse conduction, acting as power loads, wasting power and incurring damage through localized dissipation at hot spots. To prevent damage to the shaded cells by reverse current, bypass diodes 40 are commonly employed, as shown in FIG. 4. One diode per eighteen to twenty-four cells typically is used in conventional systems. When shading of one or more cells causes the bypass diode to conduct, an entire section of eighteen to twenty-four cells is bypassed, and this section contributes no power to the output. The implications of shading effects on PV system and module design are significant; indeed, field results from early residential photovoltaic installations incorporating long strings of PV cells showed a significantly lower total power yield than expected. A large portion of the power yield reduction is attributed to the problem of partial shading of the solar panel from obstructions such as clouds, power lines, utility poles, trees, and dirt.
The problem of partial shading has led to the evolution of various conventional PV system architectures as illustrated in FIGS. 5a, 5b, and 5c. With reference to FIG. 5a, most early PV system architectures included one central inverter 52 (similar in functionality to the inverter 5 shown in FIG. 1) for an entire PV array (including multiple PV modules). As discussed above, in some implementations the central inverter 52 includes an integrated MPPT apparatus to obtain as high as possible power extraction from the array. In the architecture of FIG. 5a, a number of PV modules 31 are connected in a series string 51 within the array to achieve a high output voltage. Multiple strings then are connected in parallel to increase the power output of the array. The advantage of this technique is the ability to use a single high-voltage, high-power central inverter 52 that can be made very efficient. The disadvantage is that since the respective strings of PV modules in the array are constrained to operate at the same output voltage, some strings do not operate at their maximum power point (MPP) (e.g., in the case of uneven irradiation of the modules or mismatched cells/modules). This can lead to large reductions in power yields from this type of system architecture.
FIG. 5b illustrates another conventional PV system architecture designed to mitigate problems with MPP mismatches amongst different strings of modules. In particular, the system architecture of FIG. 5b employs a string inverter concept, in which each series-string 51 of modules 31 in the array is connected to its own inverter 53. This enables each string 51 to be operated at a voltage that coincides with its MPP, and thus improves power yield. One disadvantage of this approach is the need for several inverters 53 of lower power than the single central inverter 52 shown in the system of FIG. 5a. The requirement of multiple lower power inverters typically leads to a less efficient and more expensive PV system overall. Although each string 51 of PV modules is operating at its MPP, total output power is still constrained by modules 31 with reduced output capability. In the case where a module 31 is sufficiently shaded, its bypass diodes conduct, and it absorbs power. In addition, shading of individual modules in the string 51 can lead to a situation where the MPPT apparatus (e.g., which may be integrated in each inverter 53) settles on a local optimum power point that is less than the global MPP.
To further improve power extraction from conventional PV systems, there has been movement towards other system architectures that provide MPP tracking at the individual module level. For example, with reference to FIG. 5c, another PV system architecture employs one grid-interfaced inverter 54 per module 31, which enables each module 31 to operate at its own MPP. The disadvantage of this approach is the increased number of inverters, each of which operates at low power (e.g., 100-200 W) and large voltage transformation, leading to higher total system cost and lower conversion efficiency.
As discussed above, power electronics for conventional PV systems has evolved from attempting to optimize power obtained from an entire array, to optimizing power obtained from a string of series-connected PV modules, to optimizing power obtained from individual PV modules (e.g., via maximum power point tracking, or “MPPT,” to achieve maximum power point operation).
However, even when per-module MPPT is employed in power electronics for conventional PV systems, not all of the available power may be captured from each module. FIG. 6 provides an illustrative example of the shortcomings of per-module MPPT. FIG. 6 shows a typical module 31 with seventy-two cells in series and three bypass diodes. In this example, a single cell 61 is shaded. Shading could happen for various reasons, such as due to dirt accumulation, fallen leaves, or an overhead power line. The shaded cell 61 causes the bypass diode 62 to conduct, and all twenty-four cells associated with the conducting bypass diode contribute no power to the output. Accordingly, the total output power that can be extracted is reduced by 33%. Thus, the amount of power that can be extracted from MPP operation of single PV module is often much lower than expected.
Power conversion systems providing cell-level power-point tracking have been proposed to address this issue (see, for example, R. Rohrig and J. Steger, “Circuit arrangements for photovoltaic system,” U.S. Patent Application Publication 2005/0172995, August 2005, and P. Wolfs and L. Tang, “A single cell maximum power point tracking converter without a current sensor for high performance vehicle solar arrays,” in IEEE Power Electronics Specialists Conference, pp. 165-171, 2005.) However, the methods proposed to date are inherently costly and complex, due to the use of discrete components and a relatively low level of system integration. As a consequence, such systems would be practical only in highly specialized applications.
As production capacity of PV cells has significantly increased in recent years, prices for PV cells have decreased. As a result, the cost of PV system power electronics (e.g., MPPT apparatus and inverters) required to extract the maximum power from the PV system and to interface the PV system to the grid is becoming a larger part of the overall system cost. Much attention has therefore been given to the development of power electronics that enable a cost reduction of the overall system. In addition, much research is focused on increasing the efficiency of the power processing stage, as well as on improving the power yield of the overall system.